1. Field of the Invention
This invention relates to a process for manufacturing a part made of composite material that comprises at least one radius of curvature, such as, for example, an angle bar or a stiffener with a U-shaped cross-section.
2. Description of the Related Art
FIGS. 1A and 1B show a stiffener 10 with a U-shaped cross-section. As illustrated in FIGS. 2A and 2B, this stiffener 10 can be produced by stacking pre-impregnated resin fiber layers 14 on a device 12 of convex shape.
The fiber layers 14 can be deposited using a draping machine, according to the predetermined fiber orientations.
After the deposition of the fiber layers, the unit, also covered by a bladder, is introduced into an autoclave and subjected to a cycle of temperature and pressure in such a way as to bring about the polymerization of the stiffener 10. During this polymerization stage, the pressure outside of the bladder can reach on the order of 7 to 10 bar. This pressure has the effect of causing a contraction whose purpose is to expel the air trapped between the layers and around the fibers during polymerization.
By way of indication, the contraction generates a reduction in the thickness of the part that corresponds to approximately 12 to 15% of its thickness.
When the part is supported by a device of convex shape, the contraction generates excess fiber length, more particularly at zones that have a reduced radius of curvature. Because of this excess fiber length, after polymerization, in zones that have a reduced radius of curvature, waves of fibers are produced that tend to reduce in particular the mechanical characteristics of the part.
Consequently, to limit the degradation of mechanical characteristics or other characteristics of the part, compacting of the fiber layers 14 is done during the draping phase.
According to a first operating mode that is illustrated in FIG. 2A and that is called vacuum compacting, the layers 14 of fibers deposited on the device 12 are covered with a bladder 16, and an air vacuum is applied under the bladder. This operation is renewed periodically during the draping phase, every 5 to 10 deposited layers. This operating mode makes it possible to obtain a contraction with an excess thickness before polymerization that corresponds to approximately 8% of the part's thickness that is not optimal.
According to another operating mode that is illustrated in FIG. 2B and that is called vacuum compacting with heating, a vacuum compacting is produced as described above, combined with an elevation in temperature of the material to approximately 80° C. using heating means 18. Even if this operating mode makes it possible to improve the vacuum compacting, it is not fully satisfactory because the vacuum compacting with heating makes it possible to obtain a contraction with an excess thickness before polymerization that corresponds to approximately 4% of the thickness of the part that is not optimal.
One solution for optimizing the contraction could consist in increasing the temperature during compacting. However, this solution cannot be considered because a higher temperature would lead to too high a fluidity of the resin and consequently a wringing-out of the zones that have a reduced radius of curvature because of the migration of the resin toward less compressed zones.
Another solution for optimizing the contraction could consist in increasing the frequency of vacuum compacting operations with heating. However, this solution is not satisfactory because the repetition of the heating cycles leads to increasing the enthalpy level of the resin, which is detrimental during polymerization.
In addition, the increase in the frequency of vacuum compacting operations with heating leads to reducing productivity and to increasing the production costs significantly.